1997 Heil Furnace randomely won't stay lit.

I have a 1997 Heil Furnace. It will work fine for days even weeks and then randomly will not relight but the blower stays on until I shut off at the thermostat. I leave it off for about 10 minutes then turn it back to heat and it all works fine. Like I said it might work fine for a couple days and then does the same thing. I have had service guys over, but everytime they come over it works fine for them. Any help would be appreciated I just don't have the money to replace the unit right now.